1. Field of the Invention
The present invention relates to an anodizing interconnection and to a method for fabricating a switching device using an anodization method which is used, for example, in a production process of a liquid crystal display apparatus.
2. Description of the Related Art
Active matrix type liquid crystal display apparatuses include a liquid crystal display apparatus provided with a three-terminal nonlinear device which is typified by a thin film transistor (TFT) as a switching device. Such a liquid crystal display apparatus conventionally necessitates six or more steps for depositing thin films and performing photolithography during the production process. As a result, the production process is complicated, so that the most significant challenges are to shorten the production time and to reduce the cost. Another liquid crystal display apparatus is provided with a two-terminal nonlinear device which is set into a conductive state at a predetermined voltage or more as a switching device. Such a liquid crystal display apparatus having the two-terminal nonlinear device is superior in production time and cost, and consequently has been explosively and intensively developed.
A known typical two-terminal nonlinear device is a MIM (Metal-Insulator-Metal) device. In a liquid crystal display apparatus using the MIM device as a switching device, a liquid crystal layer is provided between an active matrix substrate on which pixel electrodes and MIM devices are formed, and a counter substrate on which counter electrodes are formed. In the liquid crystal display apparatus, a voltage applied to the liquid crystal layer is abruptly changed between an on state and an off state, so that a high-contrast display can be performed even in a high duty driving which is required in response to tendencies to increase the screen size of the display apparatus and to increase the resolution.
FIGS. 11 and 12 show a construction of a one-pixel portion of a conventional active matrix substrate provided with a MIM device as a switching device. FIG. 11 is a plan view thereof and FIG. 12 is a cross-sectional view taken along a line A-A' in FIG. 11. In FIGS. 11 and 12, on a substrate 1 a signal line 2 made of tantalum and a lower electrode 3 which is branched from the signal line 2 are provided. An insulating film 4 made of tantalum pentoxide is provided so as to cover the lower electrode 3. On the insulating film 4 and the substrate 1, an upper electrode 5 made of titanium is provided. Thus, a MIM device 6 is constructed. On the upper electrode 5, a pixel electrode 7 made of ITO (Indium-Tin-Oxide) or the like is provided. The upper electrode 5 is electrically connected to the pixel electrode 7. The thus obtained active matrix substrate is attached to a counter substrate in such a manner that lines made of ITO or the like on the counter substrate are perpendicular to the signal lines 2, thereby forming a liquid crystal cell.
The conventional active matrix substrate having the above-described construction can be fabricated, for example, by the following method.
First, a thin tantalum film which will be the signal line 2 and-the lower electrode 3 is deposited so as to have a thickness of 3000 angstroms on the glass substrate 1 by sputtering or the like. The thin tantalum film is patterned into a predetermined shape by photolithography, so as to form the signal line 2 and the lower electrode 3. Thereafter, by anodization, the surface of the lower electrode 3 is anodized, so as to form an insulating film 4 made of tantalum pentoxide and having a thickness of 600 angstroms. Next, titanium which will be the upper electrode 5 is deposited so as to have a thickness of 4000 angstroms by sputtering over the entire top surface of the substrate on which the above-mentioned components have been formed. The titanium is patterned into a predetermined shape by photolithography, so as to form the upper electrode 5. In addition, a transparent conductive film made of ITO or the like is deposited and then patterned, so as to form the pixel electrode 7.
As shown in FIG. 13, especially in the conventional anodization, in the case of the anodizing interconnection for forming the insulating film 4, a formation-voltage input portion 8 and an anodizing interconnection 22 to connection terminal portions 10 to 21 of a pattern 9 to be anodized both have plane shapes. The anodization is performed by using a power source of constant current and constant voltage. As shown in FIG. 14B, a constant-current formation (in general, 1 mA/cm.sup.2 or more) is first performed. In the constant-current formation, the oxidation is performed while the formation current (I.sub.1) is set to be constant. As shown in FIG. 14A, when the voltage reaches a value corresponding to a film thickness, a constant-voltage formation is performed at the voltage (V.sub.1) for a predetermined time period.
In the above-described conventional anodizing method, there occurs a variation in display characteristics (Vop, Co, breakdown voltage, and the like) among various panel portions of a liquid crystal display apparatus. This causes a problem of nonuniformity of display.
The cause resides in the anodized film. More specifically, the thickness and the quality of the anodized film are not uniform among various panel portions. The relationship between an applied voltage V and a current I of a general two-terminal nonlinear device is expressed in the following equation. EQU I=.alpha.Vexp(.beta..multidot.V.sup.1/2) EQU .alpha.=(n.mu.q/d)exp(-.phi./kT) EQU .beta.=(1/kT)(q.sup.3 /.pi..di-elect cons..sub.1 .di-elect cons..sub.0 d).sup.1/2
n: carrier density, PA1 .mu.: carrier mobility, PA1 q: charge amount of electron, PA1 d: thickness of anodized film, PA1 .phi.: trap depth, PA1 k: Boltzmann constant PA1 T: ambient temperature, PA1 .di-elect cons..sub.1 : dielectric constant, and PA1 .di-elect cons..sub.0 : dielectric constant in vacuum.
As is apparent from the above equation, if the film thickness (d) and the film quality (n, .mu., .phi., and .di-elect cons..sub.1) are changed, the I-V characteristics (the device characteristics) are changed. This results in the variation, i.e., the nonuniformity, of the display characteristics among the various panel portions. Moreover, in the conventional anodization method, the symmetry of the device structure (the anodized film) is lost, so that the I-V characteristic of the device is not symmetric. As a result, phenomena such as residual images occur in display conditions of the liquid crystal display apparatus. In addition, because of the asymmetric I-V characteristic there arises a variation in breakdown voltages depending on the voltage applying directions, that is, the breakdown voltage in either the positive or negative direction is lower than that in the other direction. This also results in a problem in that a device defect can easily occur.